Levers for full-length MTB-style linearpull brakes need to pull more cable to actuate the brake than do levers suitable for cantilever and road bike calliper brakes. If used with a brake not designed to work with such levers, a V-brake lever brings the brake blocks up to the rim quickly. The brake will feel ‘hard’ and under-powered, however, since a lever that pulls a lot of cable has a low mechanical advantage. When used with a V-brake calliper, it translates into effective braking due to the high mechanical advantage of long V-brake arms. You don’t specify what your rear drag brake is, but it sounds like you need a lever that pulls less cable, not more. In that case, a Shimano BL-R550 brake lever, which is designed to work with calliper and cantilever brakes, will do the job if you use flat handlebars. You’ll need to stick with the V-brake lever for the front brake.
Richard Hallett, Cycle's Technical Editor